I've played Moo3 and SimCity4 and I can't recomend either.
I was a HUGE fan of moo and moo2. I was really looking forward to the third version. I bought it the first day it was out and was really let down. Its truely a nightmare to learn how to play and fully control, and its boring as heck just letting the game AI micromanage everything. I played the game through once and didn't even have the heart to play another time to see the other races. Trust me, if you loved Moo and Moo2 then stay away from this one, it'll break your heart.
I had a blast playing SimCity4, but it was buggy as heck when I was playing it. I also got it right when it was released and the game had some major issues with making your saved games corrupt or something. Once my cities got large enough to be fun I kept getting errors when trying to load them up. I would loose whole cities and it really messed up the regions. I tried several different regions and it always happened after a certian point. THere's several patches out for the game now, so they might have been able to correct this problem. And yeah, you need lots of ram and a good machine if you want to play this one. I was using an 800 MHz amd machine with 500 MB of ram and it lagged like crazy even at the lowest possible settings.
I"ve heard good things about Galactic Civ but haven't played it myself.
If you haven't played Space Empires IV, then you ought to check it out. It's one of the best space empire building games I've played. It's kinda old but I think they've released a gold version with some new mods or something.
Dave
ps. I couldn't find any new games to play either so I recently bought Starcraft again (lost my original cd's, doh) and its a blast as always.